Increases in the demand for magnetic storage capacity has resulted in a need for an increase in the areal-density (data recorded per unit area) of hard disk drives (HDDs) and other storage devices. Increases in both track density (defined by the number of tracks per unit length along the radial direction) and bit density (defined by the number of bits per unit length along the track direction) are helpful in enhancing the areal-density of the HDD. These two factors point toward decreasing bit size recorded on the disk, and one of the ways to record smaller bits is to scale down the dimension of a writer used in the recording.
In current HDD designs, a magnetic pole type writer is used to record data on a magnetic disk, which comprises grains with magnetic anisotropy along a direction perpendicular to the disk surface. Magnetic flux from the center of the writer emanates basically along the direction perpendicular to the disk surface, while that emanating from the edge makes an angle to the direction perpendicular to the disk surface. Once the field emanating from the surface of a main pole tip is large enough to overcome the “barrier,” defined mainly by the magnetic anisotropy energy (Ku) of the grain, magnetization in the grain reverses to the opposite direction and a bit may be recorded thereby. The larger the magnetic field emanating from a writer, the easier it becomes to record the bit. However, intensity of the magnetic field from the writer depends strongly on the pole geometry and dimension. In general, total magnetic flux from a writer decreases while the main pole tip surface is decreased in order to scale down to record smaller bits, which in consequence causes a lack of sufficient head field to record bits.
On the other hand, it is important to reduce the diameter of the magnetic grains of a recording layer in the magnetic disk in order to reduce the bit size and ensure higher signal-to-noise ratio (SNR) in the high areal-density region. However, a reduction of grain size makes the magnetic grain thermally unstable, which makes the increase of magnetic anisotropy energy Ku indispensable for the assurance of thermal stability. Unfortunately, increasing Ku of a magnetic grain requires more magnetic field from the writer to reverse the magnetization. Thus, recording with a writer with a scaled down main pole tip on a magnetic medium with a higher. Ku becomes more difficult.
A number of solutions that result in an improvement of the writer and the magnetic recording layer itself are already in use. Beside these solutions, “energy assisted recording” is proposed to overcome the problem of recording with limited magnetic field from a scaled down writer. The concept of this method is to supply extra energy to the medium magnetization prior to attempting to write. Among the energy assisted recording methods, thermal assisted recording (TAR) and microwave assisted magnetic recording (MAMR) probably have the most potential.
In TAR, a laser beam or high energy light, such as from a semiconductor laser diode, is guided through a wave-guide and then applied to the medium in order to increase the temperature of the medium and thermal fluctuation of magnetization. As the applied heat makes the magnetization in the recording layer fluctuate and reduces the switching field, a head field from a writer records the bit to a desired direction. In currently used TAR, a laser device is used to apply heat to the recording medium. A laser wave is guided through a wave-guide and projected on to a metal aperture, which concentrates the laser energy and transmits heat to the medium. For such a configuration, precise alignment of the laser guide and aperture is important. Variation of alignment between laser wave-guide and aperture may be a significant problem in high-volume production. Moreover, if not precisely controlled, heat passing through the wave-guide could burn the surrounding parts that comprise the head, ultimately damaging the device. Furthermore, there is a risk of corrosion of the medium itself and degradation of the medium surface due to the increase in temperature.
In MAMR, a microwave magnetic field is generated by a flux generating element positioned near to the writer main pole. As the microwave magnetic field causes oscillation of magnetization in the recording layer of the medium, a head field from a writer switches the magnetization to a desired direction. In typical MAMR, a field generating layer (FGL) is used to generate microwaves to assist recording. Stable oscillation of the FGL magnetization is a pre-requisite for MAMR and to ensure good FGL performance, several magnetic layers stacked close to the FGL are typically used. This makes the writer structure complicated and there is a risk of variation in the stability of the FGL performance, which may adversely affect the writer performance.